The Mets lost their home opener Monday, but Mayor Bill de Blasio -- attending the game to throw out the ceremonial first pitch -- declared victory in the city-versus-state battle for prekindergarten funding.

"The opening day I'm really looking forward to is the opening day of school in September," he said at Citi Field.

The city is expected this fall to launch full-day pre-K programs with the $300 million it has secured in the state budget.

De Blasio and his allies had wanted $340 million raised through a tax hike on city residents earning more than $500,000, something he pushed relentlessly. Fellow Democrat Gov. Andrew M. Cuomo had opposed the tax.

De Blasio called the state funds "the incredible victory, the historic victory," even though state legislators' rejected the tax proposal. He didn't use the word "tax" in responding to several reporters' questions about the perceived defeat.

And he could not say what amount of state aid the city will get for after-school programs, but called it "substantial."

Later, de Blasio lobbed a strike to Mets catcher Travis d'Arnaud from the edge of the pitcher's mound. The crowd booed him loudly -- twice.

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The mayor was accompanied by children from an East Harlem tutorial program near the site of the deadly March 12 building collapse. He wore a customized Mets jersey with his name and the No. 6, a nod to Rico Petrocelli, a Brooklyn native and a retired Boston Red Sox infielder. De Blasio currently lives in Brooklyn, but was raised in Massachusetts, where he came to love the Red Sox.

Before the game, de Blasio said he wouldn't be offended if he was heckled: "Sports fans have a right to express themselves any way they want."

He said he would throw the first pitch at the Yankees' opener, if he was invited. He worked several baseball analogies into his talk on universal pre-K. He said the city is getting the funds it needs, but did not rule out pursuing a tax again in future years.

"We believe, if we keep our eye on the ball, that this full commitment will be met over five years," he said. "If that's the case, we've achieved the mission. If something changes, all options are on the table."

He saw parallels in the Mets' 1986 World Series win -- clinched after they were down to their final out before Red Sox first baseman Bill Buckner's legendary error -- and his pre-K achievement.

"A lot of times people were telling us we weren't going to get pre-K and after-school done," he said. "Some people said we were down to one strike. We came back to win. "